The present invention relates to the field of semiconductor metrology and inspection. More specifically, it relates to techniques for providing and using targets for metrology and/or inspection.
Generally, the industry of semiconductor manufacturing involves highly complex techniques for fabricating integrating circuits using semiconductor materials which are layered and patterned onto a substrate, such as silicon. Due to the large scale of circuit integration and the decreasing size of semiconductor devices, the device must be defect free prior to shipment of the device to the end users or customers.
The measurement of overlay and alignment error on a wafer is one of the most critical process control techniques used in the manufacturing of integrated circuits and devices. Overlay accuracy generally pertains to the determination of how accurately a first patterned layer aligns with respect to a second patterned layer disposed above or below it. Alignment error relates to the determination of how accurately a first pattern aligns with respect to a second pattern disposed on the same layer. The terms overlay and alignment are used herein interchangeably. Presently, overlay and alignment measurements are performed via test patterns that are printed together with layers of the wafer. The images of these test patterns are captured through an imaging tool and an analysis algorithm is used to calculate the relative displacement of the patterns from the captured images.
As overlay targets become increasingly smaller, e.g., below 3 μm, proximity effects arise. By way of example, a symmetry parameter measured on a conventional target (e.g., such as a box-in-box) is independent of overlay, as opposed to smaller targets where asymmetry is a function of overlay. Of note, this correlation between overlay and the measured symmetry parameter of a smaller target is very weak for lower values of overlay. Thus, it becomes impossible to correlate a measured symmetry parameter to overlay at lower overlay values. In “In-chip overlay measurement by existing bright-field imaging optical tools,” by YI-sha Ku, Chi-Hong Tung and Smith, Nigel P., Proceedings of SPIE, 2005, the authors have proposed inducing a predetermined offset in each target to achieve a stronger correlation between a symmetry parameter measured in the target image and the actual overlay error of the target. This technique requires a model to build a relationship between the measured symmetry parameter and overlay. A problem with this technique is that the calibration for determining overlay in an x direction is not decoupled from the calibration for determining overlay in a y direction. Thus, a complex, time consuming calibration procedure for the targets in a two dimensional space is required.
Accordingly, there is a need for improved techniques for designing an overlay target that allow the determination of overlay for such target by correlating a measured symmetry parameter to overlay without requiring complex modeling procedures in both x and y directions. Simplified calibration techniques for correlated measured symmetry to overlay would also be beneficial.